Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing power in a compact system by accumulating energy in the chemical bonds of the cathode, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate and migrate towards the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction. Once a circuit is completed electrons pass back from the anode to the cathode and the Na+ (or Li+) ions travel back to the anode.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
From the prior art, for example in the Journal of Solid State Chemistry 180 (2007) 1060-1067, L. Viciu et al disclosed the synthesis, structure and basic magnetic properties of Na2Co2TeO6 and Na3Co2SbO6. Also in Dalton Trans 2012, 41, 572, Elena A. Zvereva et al disclosed the preparation, crystal structure and magnetic properties of Li3Ni2SbO6. Neither of these documents discusses the use of such compounds as electrode materials in sodium- or lithium-ion batteries.
In a first aspect, the present invention aims to provide a cost effective electrode that contains an active material that is straightforward to manufacture and easy to handle and store. A further object of the present invention is to provide an electrode that has a high initial charge capacity and which is capable of being recharged multiple times without significant loss in charge capacity.
Therefore, the present invention provides an electrode that contains an active material of the formula:AaMbXxOy                 wherein        A is one or more alkali metals selected from lithium, sodium and potassium;        M is selected from one or more transition metals and/or one or more non-transition metals and/or one or more metalloids;        X comprises one or more atoms selected from niobium, antimony, tellurium, tantalum, bismuth and selenium;        and further wherein        0<a≤6; b is in the range: 0<b≤4; x is in the range 0<x≤1 and y is in the range 2≤y≤10.        
In a preferred embodiment of an electrode of the above formula, one or more of a, b, x and y are integers, i.e. whole numbers. In an alternative embodiment, one or more of a, b, x and y are non-integers, i.e. fractions.
Preferably M comprises one or more transition metals and/or one or more non-transition metals and/or one or more metalloids selected from titanium, vanadium, chromium, molybdenum, tungsten, manganese, iron, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, magnesium, calcium, beryllium, strontium, barium, aluminium and boron, and particularly preferred is an electrode containing an active material wherein M is selected from one or more of copper, nickel, cobalt, manganese, titanium, aluminium, vanadium, magnesium and iron.
The term “metalloids” as used herein is intended to refer to elements which have both metal and non-metal characteristics, for example boron.
We have found it advantageous that the electrode contains an active material wherein at least one of the one or more transition metals has an oxidation state of +2 and at least one of the one or more non-transition metals has an oxidation state of +2.
Other suitable electrodes contain an active material wherein at least one of the one or more transition metals has an oxidation state of either +2 or +3 and at least one of the one or more non-transition metals has an oxidation state of +3.
Preferred electrodes contain an active material of the formula: AaMbSbxOy, wherein A is one or more alkali metals selected from lithium, sodium and potassium and M is one or more metals selected from cobalt, nickel, manganese, titanium, iron, copper, aluminium, vanadium and magnesium.
Alternative preferred electrodes contain an active material of the formula: AaMbTexOy, wherein A is one or more alkali metals selected from lithium, sodium and potassium and M is one or more metals selected from cobalt, nickel, manganese, titanium, iron, copper, aluminium, vanadium and magnesium.
As described above it is typical that a may be in the range 0<a≤6; b may be in the range: 0<b≤4; x may be in the range 0<x≤1 and y may be in the range 2≤y≤10. Preferably, however, a may be in the range 0<a≤5; b may be in the range 0≤b≤3; 0.5≤x≤1; and y may be in the range 2≤y≤9. Alternatively, a may be in the range 0<a≤5; b may be in the range 0<b≤2; x may be in the range 0<x≤1; and 2≤y≤8. As mentioned above, one or more of a, b, x and y may be integers or non-integers.
Extremely beneficial electrochemical results are expected for electrodes that contain one or more active materials: Na3Ni2SbO6, Na3Ni1.5Mg0.5SbO6, Na3Co2SbO6, Na3Co1.5Mg0.5SbO6, Na3Mn2SbO6, Na3Fe2SbO6, Na3Cu2SbO6, Na2AlMnSbO6, Na2AlNiSbO6, Na2VMgSbO6, NaCoSbO4, NaNiSbO4, NaMnSbO4, Na4FeSbO6, Na0.8Co0.6Sb0.4O2, Na0.8Ni0.6Sb0.4O4, Na2Ni2TeO6, Na2Co2TeO6, Na2Mn2TeO6, Na2Fe2TeO6, Na3Ni2-zMgzSbO6 (0≤z≤0.75), Li3Ni1.5Mg0.5SbO6, Li3Ni2SbO6, Li3Mn2SbO6, Li3Fe2SbO6, Li3Ni1.5Mg0.5SbO6, Li3Cu2SbO6, Li3Co2SbO6, Li2Co2TeO6, Li2Ni2TeO6, Li2Mn2TeO6, LiCoSbO4, LiNiSbO4, LiMnSbO4, Li3CuSbO5, Na4NiTeO6, Na2NiSbO5, Li2NiSbO5, Na4Fe3SbO9, Li4Fe3SbO9, Na2Fe3SbO8, Na5NiSbO6, Li5NiSbO6, Na4MnSbO6, Li4MnSbO6, Na3MnTeO6, Li3MnTeO6, Na3FeTeO6, Li3FeTeO6, Na4Fe1-z(Ni0.5Ti0.5)zSbO6 (0≤z≤1), Na4Fe0.5Ni0.25Ti0.25SbO6, Li4Fe1-z(Ni0.5Ti0.5)zSbO6 (0≤z≤1), Li4Fe0.5Ni0.25Ti0.25SbO6, Na4Fe1-z(Ni0.5Mn0.5)zSbO6 (0≤z≤1), Na4Fe0.5Ni0.25Mn0.25)zSbO6, Li4Fe1-z(Ni0.5Mn0.5)zSbO6 (0≤z≤1), Li4Fe0.5Ni0.25Mn0.25SbO6, Na5-zNi1-zFezSbO6 (0≤z≤1), Na4.5Ni0.5Fe0.5SbO6, Li5-zNi1-zFezSbO6 (0≤z≤1), Li4.5Ni0.5Fe0.5SbO6, Na3Ni1.75Zn0.25SbO6, Na3Ni1.75Cu0.25SbO6, Na3Ni1.50Mn0.50SbO6, Li4FeSbO6 and Li4NiTeO6.
It is convenient to use an electrode according to the present invention in an energy storage device, particularly an energy storage device for use as one or more of the following: a sodium and/or lithium ion and/or potassium cell, a sodium and/or lithium and/or potassium metal ion cell, a non-aqueous electrolyte sodium and/or lithium and/or potassium ion cell, an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
Electrodes according to the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices.
Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.
In a second aspect, the present invention provides a novel material of the formula: A3Ni2-zMgzSbO6, wherein A is one or more alkali metals selected from lithium, sodium and potassium and z is in the range 0<z<2.
In a third aspect, the present invention provides a novel material of the formula: Na3Mn2SbO6.
In a third aspect, the present invention provides a novel material of the formula: Na3Fe2SbO6.
The active materials of the present invention may be prepared using any known and/or convenient method. For example, the precursor materials may be heated in a furnace so as to facilitate a solid state reaction process. Further, the conversion of a sodium-ion rich material to a lithium-ion rich material may be effected using an ion exchange process.
Typical ways to achieve Na to Li ion exchange include:
1. Mixing the sodium-ion rich material with an excess of a lithium-ion material e.g. LiNO3, heating to above the melting point of LiNO3 (264° C.), cooling and then washing to remove the excess LiNO3;
2. Treating the Na-ion rich material with an aqueous solution of lithium salts, for example 1M LiCl in water; and
3. Treating the Na-ion rich material with a non-aqueous solution of lithium salts, for example LiBr in one or more aliphatic alcohols such as hexanol, propanol etc.